Fusion

What makes the Sun tick?

Fusion is the combining of two nuclei to form a new species, accompanied by the release of energy.

Fusion energy is a much-vaunted possible alternative means of generating electrical power, but the technology still illudes us. However, the fact remains that most of the universe is driven by fusion power: in stars.

Our Sun is mainly hydrogen:

Element

Percentage composition

Hydrogen

73.46%

Helium

24.85%

Oxygen

0.77%

Carbon

0.29%

Iron

0.16%

Neon

0.12%

Nitrogen

0.09%

Silicon

0.07%

Magnesium

0.05%

Sulphur

0.04%

The Sun has been burning hydrogen fuel for about 6 billion years, and is about half way through this phase of its life cycle. When it has depleted a critical amount of its hydrogen, its radiation pressure/gravity equilibrium will be lost. It will initially expand to one AU (distance Earth-Sun) radius, losing a third of its mass, before collapsing. At this stage, the star will continue with helium fusing, resulting in the fusing of carbon and oxygen. The exact evolutionary path of a star depends on its mass.

Hydrogen on Earth is mainly the $^1_1H$ isotope, meaning it has only one proton in its nucleus, with a single electron. There are rarer isotopes of $^2_1H$, deuterium, and $^3_1H$, tritium. The nucleus of a helium-4 atom is known as an alpha-particle.

Under the conditions of extreme pressure and temperature of the Sun, the following fusion reaction occurs:

$$4^1_1H {→} ^4_2He + 2^0_1e + 2^0_1ν_e + ^0_0γ$$

The combination of two nuclei with low binding energy will release energy and produce a nucleus with higher binding energy.